Problem: Ashley is 9 years older than Gabriela. Sixteen years ago, Ashley was 4 times as old as Gabriela. How old is Gabriela now?
Answer: We can use the given information to write down two equations that describe the ages of Ashley and Gabriela. Let Ashley's current age be $a$ and Gabriela's current age be $g$ The information in the first sentence can be expressed in the following equation: $a = g + 9$ Sixteen years ago, Ashley was $a - 16$ years old, and Gabriela was $g - 16$ years old. The information in the second sentence can be expressed in the following equation: $a - 16 = 4(g - 16)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $g$ , it might be easiest to use our first equation for $a$ and substitute it into our second equation. Our first equation is: $a = g + 9$ . Substituting this into our second equation, we get the equation: $(g + 9)$ $-$ $16 = 4(g - 16)$ which combines the information about $g$ from both of our original equations. Simplifying both sides of this equation, we get: $g - 7 = 4 g - 64$ Solving for $g$ , we get: $3 g = 57$ $g = 19$.